Some wireless communication systems use intermittent communication methods to reduce the power consumption of mobile stations. One example of the intermittent communication is illustrated in FIG. 17. In this example of intermittent communication, a mobile station (MS) communicates with a base station (BS) for a communication period of a predetermined length, while keeping in synchronization with the BS. During non-communication periods, the MS can save electric power by halting processors used for communication. Such an intermittent communication state may be referred to as a “sleep mode” (see, for example, International Publication Pamphlet No. WO 2005/086379).
In a cellular communication system, a scanning process is executed in order for the MS to be smoothly handed over from one cell to another. The scanning process is a measuring process for determining the next BS with which the MS will communicate and determining the quality of radio waves from a neighboring base station (NBS), which is located near a serving base station (SBS). The SBS is a base station with which the mobile station is currently communicating.
An example of the scanning process is illustrated in FIG. 18. The MS requests the SBS to provide a scan period when predetermined conditions are fulfilled, and the SBS returns a response. The scan period is defined as a duration for which the scanning process is executed. During the scan period, the MS synchronizes with a NBS, and after finishing scanning radio waves from the NBS, resumes synchronization with the SBS. Thus, during the scan period, a MS synchronizes with a NBS and does not have to communicate with the SBS.
A wireless communication system may have simultaneously two functions of intermittent communication and a scanning process. The problem with such system is that communication efficiency may be reduced. This problem is explained with reference to FIG. 19.
As depicted in FIG. 19, in a wireless communication system that has the two functions of intermittent communication and a scanning process, the scan period and the communication period during intermittent communication may overlap. The communication period during intermittent communication is a period in which a MS and a SBS communicate in order to send and receive user data like music data. However, when the communication period overlaps with the scan period, the duration of the overlap or overlap duration becomes a period during which the MS and the SBS cannot communicate. As a result, the opportunities to send and receive user data between the MS and the SBS become scarce, which leads to the decline in the communication efficiency.